Gas lift system for oil production

ABSTRACT

An improved gas lift system for use in oil production from a well bore ( 11 ) utilising a gas lift injection ( 10 ) system of the known type and further including a surface jet pump ( 16 ) downstream of the well head ( 13 ) for reducing the flowing well head pressure and capable of discharging produced fluid ( 14 ) at a pressure required by a downstream production system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims foreign priority benefits under 35 U.S.C.§119(a)-(d) to GB1202904.7 filed Feb. 20, 2012, GB1210847.8 filed 19Jun. 2012 and PCT/GB2013/050321 filed 13 Feb. 2013, which areincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gas lift system for improving oilproduction from a well.

2. Description of Related Art

The use of gas lift, involving the injection of high pressure gas atdifferent depths of an oil well, is well known as an effective way toreduce the hydrostatic head of fluids flowing through the well bore andto increase the velocity of flow in order to prevent liquids building upin the well bore.

In a typical oil production case, particularly when the gas oil ratio(GOR) of the produced fluids is low (typically below 500 Scf/bbl/day)the flow regime of the produced fluids varies within the well bore,dictated mainly by the pressure at each depth. At the lowest points nearthe production/perforation zone, the flow regime is often single phaseturbulent, bubble flow and slug flow. These types of flow regimes arisemainly because of the low velocity of gas at near bottom hole at theprevailing production pressures. These so-called erratic flow regimescause a significant loss of pressure within the well bore and result inan increase in the flowing bottom hole pressure (FBHP) and build up ofliquids because of the low velocity of gas to lift the liquids.

A rise in the FBHP will in turn restrict production. At higher levels,nearer the wellhead, the flow regime improves as the produced gas flowsat a lower pressure and therefore at a higher apparent velocity andvolumetric flow rate. For this reason the flow regime at the well boresections near the wellhead is more stable and often of dispersed bubbleflow type.

A gas lift system increases the velocity of the total gas flowingthrough the well bore and can change the flow regime within part of thewell bore to the more favorable and stable flow regimes such asdispersed bubble flow. This in turn will reduce the FBHP of the well andresults in a more stable production rate such that production canincrease.

There is an optimum rate at which gas lift is effective, beyond whicheffectiveness reduces as the high velocity of fluids will increasefrictional losses significantly; thus negating the gain from reductionin the hydrostatic head of produced fluids. The pressure of the gas liftgas is dictated by the well depth, injection depth and productionpressure at the injection point. Typical gas lift pressure has been 70to 100 barg, but this is not a limit.

There are, however, the following conditions that can arise in a gaslift system:

-   -   There is insufficient amount of lift gas available at the        desired pressure to achieve the maximum or optimum level of        reduction in the hydrostatic head of fluids and the reduction in        the FBHP for achieving maximum production.    -   The high pressure (HP) gas is not of sufficient pressure to        inject it at the lowest parts of the well bore where gas lift is        needed and is more effective.    -   The tubing size restricts the amount of lift gas as excessive        frictional losses may be experienced, negating the benefit of        lift gas.

The above mentioned scenarios restrict production and in some casescould lead to a build up of liquids within the well bore, resulting inerratic production and final seizure of production as the reservoirpressure drops with time.

There are many ways for reducing wellhead pressure, such as usingmultiphase pumps, but each solution is different and has its own uniquefeatures. The only common factor is that all boosting systems reduce thewellhead pressure. For example, WO 2011/025590 relates to the use of jetpumps downhole and is particularly for improving the production of heavyoil. In this case, by using light oil or diluents as the motive flow,the viscosity of the heavy oil is reduced which helps to reduce pressurelosses along the well bore. The system is downhole, i.e. with no use ofa surface apparatus to reduce wellhead pressure.

Surface jet pumps are known. For example, EP0717818 relates to the useof surface jet pump where flow from a high pressure oil well is used toreduce the back pressure on low pressure wells. However, in thisdocument the source of motive flow is a high pressure well and the lowpressure well is not gas lifted.

BRIEF SUMMARY OF THE INVENTION

The present invention seeks to address the identified conditions toimprove operation of a gas lift system, thereby providing acorresponding improvement in production from an oil well.

In one broad aspect of the invention there is provided an improved gaslift system for use in the production of fluid from a well boreincluding: an injector for supplying pressurized gas to the well bore; awell head; a tube (also known as “tubing”) extending from the well headinto the well bore; a plurality of openings or valves at differentdepths of the tube for permitting the ingress of injected gas to assistfluid flowing upwards through the tube by reducing the hydrostaticpressure; further including a boosting unit downstream of the well headfor reducing the flowing well head pressure and capable of dischargingproduced fluid at a pressure required by a downstream production system.

The improved gas lift system of the invention lowers the flowingwellhead pressure (FWHP) of the well in order to increase production andalso to increase the velocity of gas flowing through the well bore. Theflowing wellhead pressure (FWHP) of the well is often dictated by thedownstream production and process system, however, provision of adedicated boosting system at surface provides a clear advantage becauseit can reduce the FWHP of the well whilst delivering the produced fluidsat the pressure demanded by the downstream production system.

Preferably the boosting unit is a surface jet pump. The use of a surfacejet pump (SJP) to reduce the FWHP is a cost effective and simplesolution to rectify the problem identified by the first condition in thebackground section above and to improve production from the well.Particularly, by implementing the invention, the flow regime improveswithin the well bore because the produced fluid flows at a lowerpressure and therefore at a higher apparent velocity and volumetric flowrate.

Preferably, the motive HP flow for the SJP could be one of thefollowing: HP gas; HP oil or gas from one or more HP wells; HP liquidphase (oil or water). This HP liquid may be from an available HP sourcesuch as injection water or export oil, the pressure of which has beenboosted for other reasons.

Typical pressure of the motive high pressure flow of the surface jetpump is twice the flowing wellhead pressure, but the pressure can beseveral times this value and typically up to 70 barg or higher. The flowrate of the HP flow is also typically equal to or higher than that ofthe well flow rate to be highly effective in reducing the FWHP by afactor of two or higher.

According to the invention, the combined gas lift system (with otherwiselimited or not optimum gas lift rate) and the use of a surface jet pumpwill enhance production in a very cost effective and simple way.

In a second broad aspect of the invention there is provided an improvedgas lift system for use in the production of fluids from a well boreincluding: an injector for supplying pressurized gas to the well bore; awell head; a tube extending from the well head into the well bore; aplurality of openings or gas lift valves at different depths of the tubefor permitting the ingress of injected gas to assist fluid flowingupwards through the tube by reducing the hydrostatic pressure; furtherincluding a boosting unit upstream of the injector to boost the pressureof the lift gas.

Preferably the boosting unit is a surface jet pump configured to receivelower pressure injection gas (i.e. “lower pressure” is interpreted aslower than that ideally desired for optimum gas lift) into a firstinlet, the flow rate of gas at the first inlet being increased by a highpressure motive flow capable of being received into a second inletdownstream of a recycle loop from a compressor (e.g. an existingcompressor for provision of lift gas or for export of gas by pipeline),wherein the outlet of the surface jet pump is configured to supply gasto the inlet of the gas compression system.

This aspect of the invention addresses the problem identified where highpressure gas in the gas lift system is not of sufficient pressure toinject at the lowest parts of the well bore where gas lift is needed andis most effective. The surface jet pump acts as a “pre-boost”pressurisation means before the compressor.

The system of the invention has two main distinctions from prior artdiscussed above. The first is the fact that the low pressure well is gaslifted and the second distinction is the fact that the motive flow canbe from any source such as injection water, export oil or any highpressure liquid available. The main aspect of invention is a systemwhich combines with gas lift to increases production and compensates forcases where gas lift is not optimised, either because there isinsufficient gas or because the gas lift pressure is insufficient toinject it into the deeper part of the well where it is mostly needed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows an arrangement of an improved gas lift system according tothe invention;

FIG. 2 shows in graphical form the additional production achieved as aresult of implementing the invention;

FIG. 3 illustrates a surface jet pump suitable for use with the presentinvention; and

FIG. 4 illustrates a further improved modification of a gas lift systemaccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, increased oil production in a low GOR well isachieved by implementing a gas lift injection system 10 into a well 11such that the hydrostatic head of fluids in the extraction tube 12 isreduced. An oil/gas mixture leaves the well head 13 for downstreamprocessing via a pipeline 14.

According to known practice, a pressurized upper section 11A of the well11 is sealed from the pay zone P by a barrier 15 (through which the tube12 passes) and gas is introduced through the tube 12 wall at differentdepths via valves or apertures (not pictured in FIG. 1). The flow rateof fluid naturally rising through the tube 12 due to undergroundpressure in the pay zone P is increased by bubbles forming from theinjected gas into the tube 12.

The gas lift system as described above is generally known; however, inmany cases there may be an insufficient amount of lift gas available atthe desired pressure to achieve the maximum or optimum level ofreduction in the hydrostatic head of fluids and the reduction in theflowing bottom hole pressure for achieving maximum production. Toaddress this problem, according to the invention, a boosting unit, e.g.surface jet pump (SJP), 16 is installed downstream of the well head 13to improve the flow regime within the well bore, while reducing flowingwell head pressure. As illustrated in FIG. 1 a valve 17 may, whenclosed, divert fluid in the pipeline 14 through the surface jet pump 16or, when open, bypass the pump.

The effect of the SJP in the above described condition is an improvementin the flow regime within the well bore, a reduction in the FBHP of thewell and, ultimately, an increase in production. A suitable SJP may beas described in EP0717818; the present FIG. 3 is derived from thisdocument. The choice of the SJP system is dependent on the amount ofliquids produced with gas and whether HP gas is sufficient as the motiveflow or whether because of the rate of liquids produced the motive flowshould be HP liquid phase such as that shown in FIG. 3.

The motive (high pressure) flow 18 entering SJP 16 may be sourced from:HP gas; HP oil or gas from one or more HP wells; HP liquid phase (oil orwater). In the latter case the HP liquid may come from an available HPsource such as injection water or export oil, the pressure of which hasbeen boosted for other reasons. The advantage of using an SJP 16 toimprove the flow regime is that its high pressure fluid for operationneed not be a gas, which the gas lift system is necessarily limited to,and can be derived from many possible sources, making efficient use ofavailable resources.

Accordingly, in many cases the combination of a gas lift system atreduced rate or reduced pressure (depending on the availability andpressure of the HP gas) and an SJP system will be more cost effectiveand beneficial in maximizing production from wells (as opposed tofinding other ways to increase the injection gas pressure). The SJP alsooffers the simplest solution compared to alternative surface boostingsystems such as multiphase pumps which, if used, require a significantamount of electric power not often available on platforms or well sites.

FIG. 2 graphically illustrates the additional (i.e. rise in) production(AP) expected from a system of the invention utilizing a surface jetpump and gas lift injection system (SJP+GI), compared to gas liftinjection alone (GI). The respective curves also illustrate the optimum(peak) production, i.e. produced oil flow rate, which drops off withexcessive gas injection flow rates as mentioned previously.

Typical pressure of the motive high pressure flow of the surface jetpump is twice the flowing wellhead pressure, but the pressure can beseveral times this value and typically up to 70 barg or higher. The flowrate of the HP flow is also typically equal to or higher than that ofthe well flow rate to be highly effective in reducing the FWHP.

A further improvement is shown in FIG. 4, wherein an SJP unit 16A isinstalled within an injection gas circuit 19 to assist in raising thepressure of gas lift gas so that it can be injected effectively at thelowest part of the well to enhance gas lift and improve production. Suchan arrangement addresses the identified condition where high pressure(HP) gas is not of sufficient pressure to be injected at the lowestparts of the well bore (where gas lift is most needed and is mosteffective).

As illustrated, the SJP 16A uses as a motive flow 18 recycled gas from agas lift compressor unit 20 such that the injection gas 21 input intothe low pressure side of the SJP is “pre-pressurized” by the motive flowbefore it is further compressed by the downstream gas lift compressor20. The illustrated system boosts pressure of lift gas 10 for injectionto reach the lowest points within the well 11. It will be noted that,compared to FIG. 1, FIG. 4 does not show a barrier 15 such that liquidoil from the pay zone rises to a low level 22 in the well 11. Aspreviously mentioned, lift gas passes through the tubing, well casingannulus and the wall of tube 12 via gas lift injection valves 23. Theupper valves 23 on tube 12 represent a gas lift at limited pressure,whereas the lower valves (in this case below level 22) represent gaslift at a higher pressure. Bubbles formed in the fluid reduce thehydrostatic head pressure and enable the volumetric flow rate toincrease.

The system of FIG. 4 can be coupled with an additional SJP as in FIG. 1for further improvements in efficiency.

It will be apparent that the system of the invention can be assembledfrom available oil production components at a well site. Alternatively,the components can be manufactured for specific compatibility, i.e.tuned to cooperate with each other for best performance.

1. An improved gas lift system for use in the production of fluid from awell bore including: an injector for supplying pressurized gas to thewell bore; a well head; a tube extending from the well head into thewell bore; a plurality of openings or valves at different depths of thetube for permitting the ingress of injected gas to assist fluid flowingupwards through the tube by reducing the hydrostatic pressure; furtherincluding a boosting unit downstream of the well head for reducing theflowing well head pressure and capable of discharging produced fluid ata pressure required by a downstream production system.
 2. The gas liftsystem of claim 1 wherein the boosting unit is a surface jet pump. 3.The gas lift system of claim 2 wherein the surface jet pump utilizes amotive flow sourced from at least one of the following: high pressuregas, high pressure oil from at least one high pressure well, highpressure gas from at least one high pressure well, high pressure liquidphase oil already boosted for other reasons, and high pressure liquidphase water already boosted for other reasons.
 4. The gas lift system ofclaim 1, including a further boosting unit installed upstream of theinjector to boost the pressure of the lift gas.
 5. The gas lift systemof claim 4 wherein the further boosting unit is a surface jet pump. 6.An improved gas lift system for use in the production of fluid from awell bore including: an injector for supplying pressurized gas to thewell bore; a well head; a tube extending from the well head into thewell bore; a plurality of openings or valves at different depths of thetube for permitting the ingress of injected gas to assist fluid flowingupwards through the tube by reducing the hydrostatic pressure; furtherincluding a boosting unit upstream of the injector to boost the pressureof the lift gas.
 7. The gas lift system of claim 6 wherein the boostingunit is a surface jet pump.
 8. The gas lift system of claim 6 furtherincluding a second boosting unit downstream of the well head forreducing the flowing well head pressure and capable of dischargingproduced fluid at a pressure required by a downstream production system.9. The gas lift system of claim 8 wherein the second boosting unit is asurface jet pump.
 10. The gas lift system of claim 9 wherein the surfacejet pump is configured to receive low pressure injection gas into afirst inlet, the flow rate of gas at the first inlet being increased bya high pressure motive flow capable of being received into a secondinlet downstream of a recycle loop from a gas lift compressor, whereinan outlet of the surface jet pump is configured to supply gas to the gaslift compressor.
 11. A method of fluid extraction from a well boreutilizing a gas lift system that includes: an injector for supplyingpressurized gas to the well bore; a well head; a tube extending from thewell head into the well bore; a plurality of openings or valves atdifferent depths of the tube for permitting the ingress of injected gasto assist fluids flowing upwards through the tube by reducing thehydrostatic pressure; the method further including the provision of asurface jet pump downstream of the well head for reducing the flowingwell head pressure and capable of discharging produced fluid at apressure required by a downstream production system; wherein the surfacejet pump utilizes a motive flow sourced from one or more of thefollowing: high pressure gas; high pressure oil and/or gas from one ormore high pressure wells; high pressure liquid phase (oil or water)already boosted for other reasons.
 12. The method of claim 11 includingthe provision of a further surface jet pump upstream of the injector toboost the pressure of the lift gas.
 13. A method of fluid extractionfrom a well bore utilizing a gas lift system that includes: an injectorfor supplying pressurized gas to the well bore; a well head; a tubeextending from the well head into the well bore; a plurality of openingsor valves at different depths of the tube for permitting the ingress ofinjected gas to assist fluids flowing upwards through the tube byreducing the hydrostatic pressure; the method further including theprovision of a surface jet pump downstream of the well head for reducingthe flowing well head pressure and capable of discharging produced fluidat a pressure required by a downstream production system; wherein thesurface jet pump is configured to receive lower pressure injection gasinto a first inlet, the flow rate of gas at the first inlet beingincreased by a high pressure motive flow capable of being received intoa second inlet downstream of a recycle loop from a gas lift compressor,wherein an outlet of the surface jet pump is configured to supply gas tothe gas lift compressor.
 14. The method of claim 13 including theprovision of a second surface jet pump downstream of the well head forreducing the flowing well head pressure and capable of dischargingproduced fluid at a pressure required by a downstream production system;wherein the second surface jet pump utilizes a motive flow sourced fromone or more of the following: high pressure gas; high pressure oiland/or gas from one or more high pressure wells; high pressure liquidphase (oil or water) already boosted for other reasons.